FIG. 1 illustrates in perspective view a prior art wind turbine blade 10. The wind turbine blade 10 extends longitudinally from a generally cylindrical root 30 to a tip 34. In use, the root end 30 of the blade 10 is attached to a hub of a wind turbine (not shown). In cross-section, the blade 10 transitions from a circular profile at the root 30 to an airfoil profile at the widest part of the blade 10, which is known as the ‘shoulder’ 36. Between the shoulder 36 and the tip 34, the blade has an airfoil profile that steadily decreases in thickness and chord moving towards the tip 34.
FIG. 2 illustrates the blade 10 in cross-section and reveals that the blade 10 comprises an outer shell 12 that is fabricated from first and second half shells 14, 16. The half shells 14, 16 are laminated structures that are moulded from glass-fibre reinforced plastic (GRP). Each half shell 14, 16 comprises inner and outer skins 18, 20 with integrated load-bearing elements in the form of spar caps 22 formed from carbon fibre pultrusions arranged in a stack between the inner and outer skins 18, 20. Foam panels 24 typically fill the gaps between the load-bearing elements.
The half shells 14, 16 are moulded in separate mould halves. Once each half shell 14, 16 has been moulded, the two half shells 14, 16 are brought together by closing the mould, and the half shells 14, 16 are bonded together to form the complete blade 10.
To form a half shell 14, 16, one or more layers of dry glass-fibre fabric are placed on a mould surface of the mould half. These layers will later form the outer skin 20 of the blade 10. Structural elements, including the spar caps 22 and the foam panels 24, are then arranged on top of the outer fabric layers. One or more further layers of dry glass-fibre fabric are then placed over the structural elements, and will later form the inner skin 18.
Next, the elements of the half shell 14, 16 are covered with an airtight bag to form an evacuation chamber encapsulating the various components of the blade shell that are arranged in the mould. The chamber is evacuated using a vacuum pump, and a supply of liquid resin is connected to the chamber. The resin is introduced into the chamber and infuses between the encapsulated components.
Prior to the infusion step, the components in the blade mould are generally heated to an elevated temperature of around 30° C. This is typically achieved by using a heated blade mould, with the heat source being provided, for example, by heating elements embedded in the mould or via hot fluids. The resin is also heated to around 30° C. prior to its admission into the mould. Pre-heating the components in the mould assists the resin infusion process because it reduces heat transfer from the resin to the components in the mould, which would otherwise cause the viscosity of the resin to increase and would inhibit the flow of resin in the mould. Once the components have been infused with resin, the assembly undergoes a curing cycle to harden the resin.
The method described above is time-consuming, with a large number of components being moved into and out of the blade mould, often by hand. Any step that holds up the process is therefore highly undesirable, and steps are taken where possible to minimise the time required to lay-up, infuse and cure the components in the blade mould.
For example, in some regions of the blade 10, it is desirable to incorporate a plurality of stacked glass fibre layers into the outer skin 20. This is particularly desirable in regions of the blade 10 that need to be particularly stiff, such as at the root 30 and near the shoulder 36 of the blade 10. However, introducing multiple glass fibre layers into the mould is time-consuming, as each layer needs to be carefully aligned in the stack, and the infusion and curing time increases for thicker layers of glass fibre.
In order to reduce the lay-up time, it is known to form glass fibre pre-forms offline, which are subsequently placed in the mould. The pre-forms comprise multiple layers of dry glass fibre fabric that are cut to the required size and attached together by stitching, adhesive or another suitable means. The pre-forms are then arranged in the required locations in the mould in a single step, thereby dispensing with the time-consuming task of assembling multiple fabric layers in the mould.
However, glass fibre is a highly insulating material, and the inventors have found that glass-fibre pre-forms, being of relatively high thickness compared to a single sheet of glass fibre, take a relatively long time to heat up compared to other components in the mould. As a result, it can take a long time to heat the pre-forms in the mould to the required elevation temperature prior to resin infusion commencing.
Against this background, it is an object of the present invention to reduce the production time of a wind turbine component, such as a wind turbine blade.